Ultraviolet, infrared and terahertz photo/radiation sensors using graphene layers to enhance sensitivity

ABSTRACT

Ultraviolet (UV), Terahertz (THZ) and Infrared (IR) radiation detecting and sensing systems using graphene nanoribbons and methods to making the same. In an illustrative embodiment, the detector includes a substrate, single or multiple layers of graphene nanoribbons, and first and second conducting interconnects each in electrical communication with the graphene layers. Graphene layers are tuned to increase the temperature coefficient of resistance to increase sensitivity to IR radiation. Absorption over a wide wavelength range of 200 nm to 1 mm are possible based on the two alternative devices structures described within. These two device types are a microbolometer based graphene film where the TCR of the layer is enhanced with selected functionalization molecules. The second device structure consists of a graphene nanoribbon layers with a source and drain metal interconnect and a deposited metal of SiO2 gate which modulates the current flow across the phototransistor detector.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 15/384,248, entitled ULTRAVIOLET, INFRARED AND TERAHERTZPHOTO/RADIATION SENSORS USING GRAPHENE LAYERS TO ENHANCE SENSITIVITY,filed Dec. 19, 2016, which is a divisional of co-pending U.S. patentapplication Ser. No. 14/580,198, entitled ULTRAVIOLET, INFRARED ANDTERAHERTZ PHOTO/RADIATION SENSORS USING GRAPHENE LAYERS TO ENHANCESENSITIVITY, filed Dec. 22, 2014, now U.S. Pat. No. 9,525,136, issuedDec. 20, 2016, which is a continuation of co-pending U.S. patentapplication Ser. No. 13/308,688, entitled ULTRAVIOLET, INFRARED ANDTERAHERTZ PHOTO/RADIATION SENSORS USING GRAPHENE LAYERS TO ENHANCESENSITIVITY, filed Dec. 1, 2011, now U.S. Pat. No. 8,916,825, issuedDec. 23, 2014, the entire disclosure of which is herein incorporated byreference.

FIELD OF THE INVENTION

The present application relates generally to bundled nanotube fabricsand methods of making the same.

BACKGROUND OF THE INVENTION

Photodetectors are an integral part of optical circuits and components(for example emitters, modulators, repeaters, waveguides or fibers,reflectors, resonators, detectors, IR Focal plane arrays, UVmicrochannel arrays and THZ diode detectors, etc.) and are used for thesensing of electromagnetic radiation. There are several approaches tothese devices. Photoconducting materials, typically semiconductors, haveelectrical properties that vary when exposed to electromagneticradiation (i.e. light). One type of photoconductivity arises from thegeneration of mobile carriers (electrons or holes) during absorption ofphotons. For semiconducting materials, the absorption of a specificwavelength of light, hence photon energy, is directly proportional tothe band gap of the material (E_(g)=hn=hc/l, where E_(g) is thematerials band gap, h is Plank's constant (4.136×10⁻¹⁵ eVs), c is thespeed of light in a vacuum (2.998×10¹⁰ cm/s) and 1 is the wavelength ofthe radiation). If the band gap energy is measured in eV (electronVolts) and the wavelength in micrometers, the above equation reduces toE_(g)=1.24/1. A photodiode (i.e. p-n diode, p-i-n photodiode, avalanchephotodiode, etc.) is the most commonly employed type of photoconductor.

Light detection is ideally suited for direct band gap semiconductorssuch as Ge and GaAs. However, indirect band gap semiconductors (where anadditional phonon energy is needed to excite an electron from thevalence band to the conduction band), such as Silicon, are also used asphotodetectors. A widely known type of photodetectors is the solar cell,which uses a simple p-n diode or Schottky barrier to detect impingingphotons. Besides silicon, most photodetectors disadvantageously do notintegrate with existing microelectronics technology, usually detect onlya specific wavelength (i.e. 1.1 mm for Si, 0.87 mm for GaAs, 0.414 mmfor a-SiC and 1.89 mm for Ge), and require multiple detectors to detecta broad band of wavelengths (hence photon energy).

Besides photodiodes, there are other types of photodetectors that do notrely on the generation of current through the excitation of electrons(or holes). One type of detector is the bolometer. Bolometers operate byabsorbing radiation, which in turn raises the temperature of thematerial and hence alters the resistance of the material. For usefulbackground information on bolometers and semiconductor devices, refer toKwok K. NG, “Complete Guide to Semiconductor Devices,” IEEE Press, JohnWiley & Sons, 2002, pages 532-533. Bolometers can be constructed frommetallic, metallic-oxides or semiconducting materials such as vanadiumoxide and amorphous silicon. Since bolometers detect a broad range ofradiation above a few microns, bolometers are typically thermallystabilized to reduce the possibility of detection of blackbody radiationthat is emitted from the detector material, which leads to a highbackground noise. Unlike other detector technologies, IR microbolometerdetectors and arrays advantageously do not require cooling to cryogenictemperatures unlike the other detector technologies discussed.

Typical band-gaps for carbon nanotubes (CNTs) range from approximately0.6-1.2 eV, depending on the diameter of the CNT, where the band gap isproportional to the inverse diameter of the nanotube. These energiescorrelate to the ability of the nanotubes to detect radiation in thenear IR range. Since nanotubes also generate heat and phonons by severalprocesses (e.g., injection of electrons, impinging with radiation,etc.), a CNT fabric is also ideally suited as an IR detector. Forgraphene, which has a zero electron volt band gap, high mobilities(approximately 100,000 cm2/Vs) and carrier saturation velocities on theorder of approximately 5×10E7 cm/s, the nanoribbons can serve as eitherphotodetectors or a microbolometer through modulation of the temperaturecoefficient of resistance of the graphene layer(s).

An existing prior art microbolometer utilizes vanadium oxide as theelement which changes impedance for incoming IR radiation. Typically 2%per degree C. is the highest thermal coefficient of resistanceachievable. This performance is limited by 1/f noise and the basicphysical properties of the vanadium oxide film. The VOx based microbolometer is fabricated on top of the CMOS readout circuit, whichprovides a cost benefit.

Accordingly, it is desirable to provide carbon nanotube based UI, IR andTHZ radiation and light detecting systems to enhance overall sensitivityof the system.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byproviding a light detector includes a single graphene layer or multiplelayers article in electrical communication with a first and a secondcontact; and a detection circuit in electrical communication with thefirst and second contacts. The detection circuit provides electricaloutputs for sufficient light detection from the nanotube article in theproximity of the predefined region by use of preamplification.

In accordance with an illustrative embodiment, the predefined regionwhere graphene layer(s) are situated on a cantilever beam that providesthermal isolation from the surrounding environment.

According to an illustrative embodiment, the predefined region isbetween two electrical contacts. These electrical contacts provideelectrical communication but also are designed for maximum thermalisolation. In addition in order to create low electrical resistancegraphene to interconnect connection, the use of Palladium or platinum isrequired to enhance pi bond connects in the graphene phase and the metalinterconnects.

In accordance with an illustrative embodiment, the graphene baseddetector invention light detection arrays can be integrated withsemiconductor circuits including CMOS circuits which provide pixel arrayx-y controls, pre-amplification of the modulated resistance signal fromthe IR detector and the conversion of the analog signal to digital.

According to an illustrative embodiment, the graphene nanoribbon film(s)increase the temperature coefficient of resistance from state of the artof 0.025 per degree Centigrade to in excess of 0.04% per degreecentigrade.

In accordance with an illustrative embodiment, the graphene basedmicrobolometer detects light by resistance changes in the fabric due toheating.

According to an illustrative embodiment, the IR detector no longersuffers from the Nyquist frequency limitation. This is due to the factthat the Nyquist frequency limitation is due to the presence of 1/f orflicker noise. The use of graphene ribbons exhibit non measurable noisesources, and thus the IR detector no longer suffers from the Nyquistfrequency limitation. Within optical systems with f1, the elimination ofNyquist limited behavior significantly improves the performance of IRdetection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention description below refers to the accompanying drawings, ofwhich:

FIG. 1 is a schematic diagram of a microbolometer detecting elementaccording to an illustrative embodiment employing a graphene sensingelement fabricated on a generic CMOS wafer process;

FIG. 2A is a schematic diagram of the resulting structure after a firststep is performed in fabrication of a graphene based IR detector, inwhich a film is deposited on a substrate and standard photolithographycreates a hole over the tungsten (W) plugs, according to theillustrative embodiment;

FIG. 2B is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in which a thinfilm of Cu is deposited, according to the illustrative embodiment;

FIG. 2C is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in which alayer of amorphous silicon is deposited, according to the illustrativeembodiment;

FIG. 2D is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in which thelayer of amorphous silicon is planarized using chemical-mechanicalpolishing, according to the illustrative embodiment;

FIG. 2E is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in whichcontact holes are provided through the amorphous silicon and siliconoxide layers, thereby clearing the material down to the underlyingtungsten (W) plus, according to the illustrative embodiment;

FIG. 2F is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in whichstandard CMOS interconnect metallurgy is deposited, according to theillustrative embodiment;

FIG. 2G is a schematic diagram of the resulting structure after anotherstep is performed in the fabrication of the IR detector, in which agraphene layer is deposited, according to the illustrative embodiment;

FIG. 2H is a schematic diagram of the resulting IR detector after thefinal step is performed in the fabrication of the IR detector, in whichthe graphene layer is masked off to create the image detector designdesired and the amorphous silicon in the cavity is etched, according tothe illustrative embodiment;

FIG. 2I is a flow chart of a procedure for the fabrication of graphenefrom carbon nanotubes, according to the illustrative embodiment;

FIG. 3 is a perspective view of the fully assembled graphene basedmicrobolometer, in accordance with an illustrative embodiment;

FIG. 4 is a top view of an array of graphene based microbolometers, inaccordance with the illustrative embodiments;

FIG. 5 is a schematic diagram of the CMOS readout circuit for thegraphene IR detector, in accordance with the illustrative embodiments;

FIG. 6 is a graphical diagram of the measured film resistance of agraphene layer versus temperature, according to the illustrativeembodiment;

FIG. 7A is a schematic diagram of a photo-field effect transistor devicestructure incorporating a graphene layer or multilayer, in accordancewith the illustrative embodiments;

FIG. 7B is a band gap diagram of the structure of FIG. 7A, according tothe illustrative embodiments;

FIG. 8A is a graphical diagram of the Ultraviolet (UV) absorption of agraphene layer, according to the illustrative embodiments;

FIG. 8B is a graphical diagram of the Infrared (IR) absorption of agraphene layer, according to the illustrative embodiments; and

FIG. 8C is a graphical diagram of the Terahertz (THZ) absorption of agraphene layer, according to the illustrative embodiments.

DETAILED DESCRIPTION

Devices including graphene single layers or multilayers suspended overgaps (for example, gaps of approximately 50-250 nm) can be employed asInfrared (IR) radiation detectors. In addition, the application ofgraphene single layer or multilayers on a thermally isolated cantileverbeam can be employed as an IR radiation detector. One possible techniquethat can be used to detect electromagnetic radiation is a resistive typemicrobolometer that changes its electrical resistance as its temperaturerises due to the absorption of electromagnetic radiation.

Graphene based detectors have several important and unique features thatare not available with existing technologies. First, arrays of thesenanotube light detectors can be formed using patterning technology atminimum dimensions of the lithography node used or dictated by thedemands of the optical imaging system. It is possible to create 25, 17,or 8 micron square or less detectors limited only by photolithographytechniques.

Although most of the illustrative embodiments herein are described asthough the fabric is made of nanotubes of the same type, (e.g., allsingle-walled), the fabrics can be composed of all multi-walledstructures or of a combination of single- and multi-walled structureswhich are processed into graphene nanoribbons.

Illustrative embodiments of the invention allow integration at a levelof one light detector per ten or less transistors at the minimumdimension of a given lithography node or the integration of large arraysthat are addressed by CMOS pre-amplification or readout and logiccircuits. Previously only discrete components, such as silicon p-ndiodes, could be used as light detectors for optoelectronic circuits.Other types of detectors require complex and difficult fabricationtechniques such as flip-chip processes to integrate with siliconsemiconductor technology. Because CNT light sensors can be integrated toform VLSI arrays, which allows for optical interconnects having onelight detector per transistor (or waveguide, depending on function), thefabrication of ultra-dense optical circuits is possible.

According to illustrative embodiments, light detecting elements have asuspended region of nanofabric overlying a substrate material. FIG. 1shows a schematic diagram of an IR detector having a graphene basedfabric sensing element fabricated on a generic CMOS wafer. The IRdetector incorporates a graphene based fabric sensing element forperforming the infrared detection. The IR detector 100 includes aconventional P-N junction substrate 101, which is part of the overallCMOS logic 110. The substrate 101 can comprise silicon using a Bridgmanfloat zone technique. There is a film 120 deposited on the substrate 101as well as the graphene nanoribbon IR sensors 130, for performing the IRdetection. The film 120 can comprise a silicon oxide layer based uponthe absorption frequency for the type of device. The IR detector 101 isfabricated in accordance with the procedures outlined in FIGS. 2Athrough 2J.

Light detectors can be constructed using suspended or non-suspendednanotube-based fabrics in combination with appropriate substrates.Fabrication techniques to develop such horizontally- andvertically-disposed fabrics and devices composed of nanotube fabricswhich comprise redundant conducting nanotubes may be created via CVD, orby room temperature operations as described herein. For usefulbackground material on fabrication of carbon nanotubes, refer to U.S.Pat. No. 6,706,402, and published PCT Application No. WO 01/03208, whichare expressly incorporated by reference herein. Such detectors can bepart of a scheme involving signal transmission or use in a display.

The substrate material 101 can be an insulator such as one describedhereinabove or can be a semiconductor (such as, but not limited to, Si(single crystal, polycrystalline and amorphous), Ge, SiGe, SiC, Diamond,GaN, GaAs, GaP, AlGaAs, InP, GaP, CdTe, AlN, InAs, Al_(x)In_(1-x)P, andother III-V and II-VI semiconductors) or a conductor (such as, but notlimited to, Al, Cu, W, Al(<1% Cu), Co, Ti, Ta, W, Ni, Mo, Pd, Pt, TiW,Ru, CoSi_(x), WSi₂, TiSi_(x), TaN, TiN, TiAlN, RuN, RuO, PtSi, Pd₂Si,MoSi₂, NiSi_(x)). The substrate material systems can be chosen forcircuitry technologies and light absorption considerations, the graphenefabric and associated microbolometer structure processing are compatiblewith all of these materials. The suspended region (see region 272 ofFIG. 2H) defines the electromagnetic sensing region of the detectingelement 100.

The light detection from the detecting element 130 is controlled bydriving circuitry. Refer to FIG. 5 for a diagram of exemplary drivingcircuitry 510, 520, 521 and 530.

The layers may have thickness of about 1 nm or less, i.e., the thicknessof a given nanotube, or may be composed of several layers of overlappinggraphene layers to create a multilayered film of >>10 nm. The nanotubefabric can be grown or deposited on a surface, as described above, toform a contiguous film of a given density. This film can then bepatterned to a minimum feature size of 1 nm, corresponding to a singlenanotube left in the article. More typically, the lower dimension sizesof the nanotube film are a consequence of lithographic technologylimitations and not any limitations inherent in the preferredembodiments of the invention. After patterning, graphene layers can befurther integrated with metal interconnects and dielectric passivationlayers to create a circuit element. Refer to FIGS. 2A-2I for a detaileddescription of the fabrication techniques.

Light detectors can be constructed using suspended or non-suspendednanotube-based fabrics in combination with appropriate substrates.Fabrication techniques to develop such horizontally- andvertically-disposed fabrics and devices composed of nanotube fabricswhich comprise redundant conducting nanotubes may be created via CVD, orby room temperature operations as described herein and others known inthe art. Detectors can be part of a system involving signal transmissionor use in a display.

Light can be impinged on the open area of these bundled carbon nanotubefabrics to cause the generation of heat in the fabric, such as abolometer. Or in the case of the phototransistor based photodetectorsthe absorbed light carriers

Suspended graphene layers are ideal structures for monolayered fabrics,which have a high porosity. Since the substrate may influence thedetection of radiation, the suspended region should diminish anydisadvantageous substrate thermal isolation effects.

Reference is now made to FIGS. 2A-2H, showing the various stages of thefabrication procedure for an IR detector incorporating graphene layers.As shown in FIG. 2A, using standard CMOS microelectronics processingtechniques, a deposited silicon oxide film 201 is deposited on thesubstrate 202. A standard photolithography method, known in the art, isused to create a hole 205 over the tungsten (W) plugs. The Tungstenplugs 203 serve as interconnects to the underlying CMOSpre-amplification circuitry 204. Refer to FIG. 5 for a diagram of anexemplary CMOS circuitry. The next step, as shown in FIG. 2B, is to useelectron beam evaporation or Direct current sputtering to deposit a thinfilm of Copper (Cu) 211 which serves as an IR photon reflector.

As shown in FIG. 2C, in the next step of the fabrication process a layerof amorphous silicon 220 is deposited and planarized usingchemical-mechanical polishing to result in the amorphous silicon 230 ofFIG. 2D. The next step is use standard photolithography techniques usinga photoresist stencil and reactive ion etching to etch contacts holes240 through the amorphous silicon and silicon oxide layers clearing thematerial down to the underlying tungsten plugs which serve asinterconnects to the underlying CMOS circuitry, as shown in FIG. 2E. Thenext step is to use direct current sputtering to deposit standard CMOSinterconnect metallurgy, aluminum-copper thin films 250. Standardphotolithographic/dry etch techniques are used to delineate theinterconnect structures, as shown in FIG. 2F. The next step, as shown inFIG. 2G, is to deposit graphene layer(s) 260. The final steps are tomask off the graphene fabric and employ standard photolithographicmethods to create the image the detector design required. Finally usingXeFl2 (Xenon Difluoride) etching, or other techniques known in the art,the amorphous silicon in the cavity is etched and a gap or cavity 272 iscreated, which results in the fully fabricated device as shown in FIG.2H having a suspended region of nanofabric 270 overlying the gap 272.Also refer to FIG. 3 for a perspective view of the fully fabricateddevice.

Reference is made to FIG. 2I, illustrating a flow chart of a procedurefor the fabrication of graphene from carbon nanotubes. It is assumedthroughout that, with the exception of the Zip Plasma Etch portion,standard microelectronic processing and processing known in the art isutilized. At step 274, the single wall carbon nanotubes are suspended ina aqueous or organic solvent solution. The concentration of the singlewall nanotubes is optimized to maximize surface coverage of the coatedsurface. There are two techniques for optimization of surface coverage:one where the unzipped CNTs cover the entire surface and another wherelayers of graphene are deposited on top of each other. Both conditionsare conducive to detector functionality after optimization forelectrolyte penetration. The interconnect metals are deposited at step273, and the metals can include aluminum, aluminum copper, copper,palladium and platinum.

Steps 275, 276 and 277 in the process 285 use Semiconductor Industrystandard photoresist apply, bake and dry equipment. These correspond tothe steps shown, respectively, in FIGS. 2A and 2B as elements 201through 205. An optimization process using design of experimentsmethodology optimizes for graphene and adhesion to the underlyingsurface. At steps 275, 276 and 277, quantity, chuck rotation speed andbake temperatures are optimized for surface coverage and thickness.

Using standard photoresist and lithographic techniques the features ofthe detector design are created in photoresist at step 278. Afterphotoresist dispense, the entire wafer is exposed to an oxygen plasma atstep 279, the areas of CNTs not covered by the photoresist will beremoved by the plasma. In the next step the photoresist is removed andthe graphene exposed structures are baked at step 280.

The plasma zip portion of the process at step 281 uses hydrogen andhydrogen compounds in a low temperature plasma environment. Varioustypes of plasma equipment can be utilized, glow discharge, diode,reactive ion etch, and Electron-cyclonic resonance configurations. Thepressure and incident power regimes are optimized for each type ofplasma reactor configuration for optimal performance. Pressure regimesare between approximately 10 mTorr and 300 mTorr with incident power andprocess pressure requiring process optimization for each reactor typeand can vary within the scope of ordinary skill.

One indicator for process optimization is to use CNT based field effecttransistors (for example as shown in FIG. 7A) and measure the ratio ofthe current on over the current off. After the optimized space isdetermined then the process is further optimized by examining thegraphene sheets with Transmission Electron Microscope for defects,electronic mobility and degree of CNT rupture.

After the graphene sheets are created then the space is deposited atstep 282, the second graphene electrode is fabricated at step 283 byrepeating steps 274-281, and the final interconnect metal is depositedand etched at step 284. At this point in the process the fabricated IRdetector is ready for use and/or testing. In further illustrativeembodiments, the wafers are ready for further device processing for RF,analog, digital and power microelectronics.

Reference is now made to FIG. 3, showing a perspective view of a fullyassembled graphene based microbolometer, according to an illustrativeembodiment. A graphene based microbolometer structure 300 is shown,having readout locations 310. The structure 300 includes a graphenenanoribbon fabric 312 suspended above the substrate 313, in accordancewith the techniques described herein and readily apparent to thosehaving ordinary skill. The thermally isolated cantilever structure 314is also shown, as well as the connection to tungsten (W) plugs 316. Anarray of graphene nanoribbon based microbolometers is shown in the topview of FIG. 4, in accordance with the illustrative embodiments. Thearray 400 of microbolometers includes a plurality of microbolometers401, 402, 403, 404, 405, 406, 407, 408, 409, 410, 411, 412, 413, and414.

FIG. 5 is a schematic diagram of an exemplary CMOS readout circuit forthe graphene nanoribbon IR detector in accordance with the illustrativeembodiments. As shown in the diagram 500, there is a common half circuit510 operatively connected to a unit cell circuit 520 which includes theIR detector 521. A dynamic discharging output stage circuit 530 isoperatively connected to the unit cell circuit 520 to define the overallCMOS readout circuit 500.

FIG. 6 is a graphical diagram of the measured film resistance of thegraphene nanoribbon fabric versus the temperature, according to theillustrative embodiments. As described hereinabove, the electricalresistance of the microbolometers changes as the temperature rises dueto the absorption of electromagnetic radiation in the fabric. This isillustrated in the graphical diagram 610 of FIG. 6. As shown, duringboth the first pass 621 and the second pass 622, as the temperatureincreases, the resistance of the microbolometer changes. Accordingly,this allows the structure to be employed as an IR detector in accordancewith the illustrative embodiments.

Reference is now made to FIG. 7A showing a schematic diagram of aphoto-field effect transistor device structure incorporating a graphenelayer or multilayer, according to an illustrative embodiment. As shown,the fully assembled IR detector is operatively connected to source andground where appropriate to provide a photo-field effect transistor. Asource 701 and drain contact 702 are deposited and etched onto a siliconoxide layer 703 that is deposited on a substrate 704, such as silicon,GaAs, or other compound semiconductors. Graphene layers 705 arefabricated and deposited on the silicon oxide layer 703. A CMOScompatible thin film metal 706 is deposited, such as palladium orplatinum, upon which the source and drain contacts 701, 702 arefabricated.

A metal or oxide gate electrode 707 is fabricated on top of the graphenelayer or layers. The gate electrode 707 can comprise a deposited metalof SiO2, which modulates the current flow across the phototransistordetector. In some embodiments, it may be necessary to fabricate a space708 between the top of the graphene and the bottom of the metal orsilicon oxide gate electrode.

FIG. 7B shows a band gap diagram of the photo-field effect transistor ofFIG. 7A. As shown, with the initiation of photon illumination, electronsmove either towards the Vd level or into the conduction band. Holes movetoward the Vg level, thereby creating a depletion region in the p-njunction.

Reference is made to FIGS. 8A, 8B and 8C showing, respectively,graphical diagrams of Ultraviolet (UV), Infrared (IR) and Terahertz(THZ) absorption of a graphene layer. FIG. 8A is a graphical diagramshowing the UV absorption of a graphene layer. Note that the UVabsorption increases significantly as the excitation energy (eV) isincreased. This data demonstrate the ability of the graphene layer toabsorb radiation over the bands of the UV range. FIG. 8B shows agraphical diagram of the IR absorption of a graphene layer. As shown, agraphene layer of 10 microns has significantly higher absorption than agraphene layer of 1.0 microns. This increased absorption demonstratesthat multiple layers of graphene thereby increases the absolute amountof photon absorption. This enables the tuning of devices to maximizephoton absorption as part of the device design. FIG. 8C is a graphicaldiagram of the THZ absorption of graphene layers, showing thedetectivity as a function of the number of graphene layers. Note thatthe THZ absorption increases as the number of graphene layers increases.

The teachings herein can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiments are therefore to be considered illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description.

The foregoing has been a detailed description of illustrativeembodiments of the invention. Various modifications and additions can bemade without departing from the spirit and scope of this invention. Eachof the various embodiments described above may be combined with otherdescribed embodiments in order to provide multiple features.Furthermore, while the foregoing describes a number of separateembodiments of the apparatus and method of the present invention, whathas been described herein is merely illustrative of the application ofthe principles of the present invention. For example, the illustrativeembodiments can include additional layers to perform further functionsor enhance existing, described functions. Likewise, the electricalconnectivity of the cell structure with other cells in an array and/oran external conduit is expressly contemplated and highly variable withinordinary skill. Additionally, it is expressly contemplated thatsingle-wall nanotubes, multi-wall nanotubes, and any combinationthereof, can be employed. More generally, while some ranges of layerthickness and illustrative materials are described herein, these rangesare highly variable. IT is expressly contemplated that additionallayers, layers having differing thicknesses and/or material choices canbe provided to achieve the functional advantages described herein. Inaddition, directional and locational terms such as “top,” “bottom,”“center,” “front,” “back,” “on,” “under,” “above,” and “below” should betaken as relative conventions only, and are not absolute. Furthermore,it is expressly contemplated that various semiconductor and thin filmfabrication techniques can be employed to form the structures describedherein. Accordingly, this description is meant to be taken only by wayof example, and not to otherwise limit the scope of this invention.

What is claimed is:
 1. A method for converting carbon nanotubes intographene sheets, comprising: etching, using hydrogen plasma at atemperature between 32-200 degrees Fahrenheit and at least one of glowdischarge, diode, reactive ion etch, or electron-cyclonic resonance,latent defects in and thereby carbon nanotubes split the carbonnanotubes in a longitudinal direction resulting in graphene sheets. 2.The method of claim 1 wherein a diameter of the carbon nanotubes is nota factor in the 1 to 5 nm range.
 3. The method of claim 1 wherein thehydrogen plasma occurs during gas flow uniformity and radio-frequencyuniformity that results in substrate etch uniformity.
 4. A method offabricating graphene sheets from single wall carbon nanotubes,comprising: suspending the single wall carbon nanotubes in an aqueous ororganic solvent solution; etching, using hydrogen plasma at atemperature between 32-200 degrees Fahrenheit and at least one of glowdischarge, diode, reactive ion etch, or electron-cyclonic resonance,latent defects in carbon nanotubes and thereby split the single carbonnanotubes in a longitudinal direction resulting in the graphene sheets.5. The method of claim 4 wherein a diameter of the carbon nanotubes isnot a factor in the 1 to 5 nm range.
 6. The method of claim 4 whereinthe hydrogen plasma occurs during gas flow uniformity andradio-frequency uniformity that results in substrate etch uniformity.